Cleanup of association between metrics and collection modules

• Got rid of the metric_id field in StartCollectingRequest. Collection module implementations now do not need to care about metrics
• Moved the metric_id field in CollectionModules to metrics. It contains a list of metrics the collection module feels responsible for. This is a transitionary step and it will be removed in the future
• Removed gathered_using from the evidence struct, as the collection module implementation does not care and does not know about metrics